1. Field of the Invention
The present invention relates to the field of digital encoding of pictorial information for use in forming color reproductions on display or printing systems.
2. Description Relative to the Prior Art
With the advent of printing using digital technology, images may be printed, by rendering the image into a set of pixels. In pure binary printers, the pixel is either on (black) or off (white). Such techniques are well suited to reproducing text because the sizes of the individual pixels that make up the symbols are much smaller than the symbols. Thus, the human eye sees the text as a continuous image even though it is a collection of closely spaced dots.
However, most binary print engines and particularly electrophotographic print engines do not provide acceptable levels of gray for other images, such as photographs. Those skilled in the art have used halftone dots to emulate grayscale for reproducing images with continuous tones. One reason for this is that the particles used for forming the printed dots may be larger than is desirable even if the printing system were suited to printing very small binary pixels.
In the area of digital printing (the term “printing” is used to encompass both printing and displaying throughout), gray level has been achieved in a number of different ways. The representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been well known to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye give a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density. The effect is such that a group of dots and dot-less blank spots, when seen by the eye, is a rendition of an intermediate color tone or density between the color of the initial paper stock, usually white, and total ink coverage, or solid density halftone dot. It is conventional to arrange the dots in rows, where the distance between rows is known as line spacing, and determines the number of lines per inch (lpi). In the ensuing paragraphs, discussions will be made in terms of white paper stock; it is understood that white paper stock is used as an illustration and not as a limitation of the invention and that other media may be used such as plastics, textiles, coated papers, metals, wood, edible articles, etc.
Continuous tone images contain an apparent continuum of gray levels. Some scenes, when viewed by humans, may require more than two hundred and fifty six discrete gray levels for each color to give the appearance of a continuum of gray levels from one shade to another. Halftone pictorial or graphical images lower the high contrast between the paper stock and toned image and thereby create a more visually pleasing image. As an approximation to continuous tone images, pictorial imagery has been represented via binary halftone technologies. In order to record or display halftone images one picture element of the recording or display surface consists of a j×k matrix or cell of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements (pixels or pels) or leaving them blank, in other words, by suitably distributing the printed marks within each cell.
Another method of producing gray levels, is provided by gray level printing. In such a method, each pixel has the capability to render several different dot sizes. In certain electrophotographic printing systems, for example, the dot size for a pixel is a function of the exposure time provided an LED element corresponding to that pixel. The longer the exposure time, the more toner is attracted to that particular pixel.
There are two major concerns in rendering a continuous tone image for printing: (1) the resolution of image details, and (2) the reproduction of gray scales. These two fundamental factors compete with each other in a binary representation scheme. The more gray levels that are rendered, the larger is a halftone cell. Consequently, coarse halftone line screens are provided, with the attendant poor image appearance. Hence, compromises made in rendering between the selection of line resolution in gray scale and binary halftone printing. However, with gray level halftone printing, one can satisfy both resolution and gray level requirements. In gray level printing, the same number of addressable dots are present, and there is attached a choice of dot sizes from one dot size of 1 bit/pixel to for example 255 different dot-sizes of 8 bits/pixel. Although providing higher image quality with respect to line resolution and tone scales, gray level halftone presents its own dot rendering issues.
A number of different dot layouts are possible to build gray level dots from a cell template. These gray level dots are the digital representation of the gray level screening, and must be realized through a printing process. It is desirable in gray level screening to layout the dots with the printing process characteristics built into it such that the appearance of the dots are pleasing to the eye: less grainy, stable, less artifacts, less texture (i.e., visible screen and its microstructure).
An example of a line screen designed for gray scale rendering is disclosed in U.S. Pat. No. 5,258,850. The arrangement of pixels within a halftone cell is such that growth within a cell to represent increases in density is accomplished through arranging the pixels along lines of growth. Another example of a halftone cell is that shown in U.S. Pat. No. 5,258,849, which features growth of density within a halftone cell by gradual enlargement about a central area within the cell. The halftone cells disclosed in the above two patents are notable in that the pixels we need within each cell may vary in density. This substantially increases the number of gray levels that may be represented by the overall halftone cell from that where the pixels can only be rendered as a binary representation (either black or white with no distinction regarding size). The combination of cells represents a halftone screen.
Color printing on halftone printers involves the formation of color separations as halftone screens for each color, which is to be used to form a color image. The halftone screens are laid down on a predetermined overlapping relationship to each other, which results in generation of the desired color image. A well-known problem when overlapping two or more halftone screens is the possibility of developing a moiré pattern or other form of interference, when the screens are not properly positioned. To avoid the moiré or other undesirable patterns, precise angle combinations of the screens are required. It is known that increasing the difference in angle of two overlaid dot screens will result in a smaller pattern, making the pattern less apparent. However, the prior art teaches, see for example U.S. Pat. No. 6,307,645, the largest possible angle difference between two overlaid screens should be no more than 45° because a 90° screen is essentially the same as 0°, just as a 135° screen is the same as a 45° screen even in the context of attempting to reduce moiré with asymmetrical dots.
In color image printing it has been common practice to use at least three process colors and in more cases three process colors and black. In the case of four-color printing the printing industry has generated a standardized combination of four halftone angles. In particular and with reference to FIG. 1, the cyan halftone screen is located at 15°, the black halftone screen at 45°, the magenta halftone screen at 75° and the yellow halftone screen at 0°. Since yellow is the lightest and least noticeable color, it can be set at 0°, even though 0° is a highly noticeable angle, and that is only 15° from the nearest neighbor. In some embodiments, the cyan halftone screen is known to be set at 105°, however, with symmetrical dots this is substantially the same as 15°, and the prior art recognizes that even with asymmetrical dots it does not make a large difference.
When the four process colors using the above halftone screen angle combinations are overlaid, the resulting moiré or other interference patterns are as small as possible. A visually pleasing rosette structure is formed when the individual dots grains are oriented 30° apart. The traditional graphics art printing has been made using this 15°/45°/75° angle screen design to form a balanced rosette structure. In the CMYK four-color printing process, the yellow screen is usually designed at 0° or 45°. However, the moiré pattern resulting from the interaction between the yellow screen and the other three individual screens due to miss-registration is not as visually pleasing as a 30° moiré pattern (rosette structure). Yellow is a light color, so this additional moiré is usually acceptable and not very noticeable in most CMYK four-color printing systems. However, careful examination of prints shows that this yellow moiré pattern can be seen in certain composite colors. Where additional colors are used such as in a hi-fi color (for example, a five-color) printing system, there is a need to design a fifth screen on top of the original well-balanced CMYK screen set. This is particularly true where the fifth color screen is blue, the complementary color of yellow, and the blue color screen is placed at the same screen angle and screen frequency as the yellow color screen. The unpleasant moiré, which was not noticeable in the yellow color, will now show up in the blue color.
It is thus known that many color printing systems will include five or more printing units using different color colorants. Attempting to incorporate these additional colors is noted to be difficult, especially if each color must have a halftone screen with a unique halftone angle. Particularly, once there are more than four screens with attendant screen angles, which must be laid down, the patterning problems discussed above, are greatly increased. It would thus be desirable to provide color screen sets for printing which minimize the unpleasant moiré patterns formed including those caused by the interactions of the yellow screen.